Maraging steel is a type of high-strength, low-carbon martensitic steel which contains relatively high amounts of nickel and in which hardening precipitates are formed by aging. Due, in part, to its exceptional mechanical properties, such as high tensile strength, maraging steel has been employed in various commercial and military applications. Commercial applications include, for example, aircraft forgings. Certain maraging steels have also been found to possess particularly good properties for military applications, such as for use in armor plate, and have been shown to exhibit good resistance to penetration by projectiles.
Historically, armor plate has been produced from various materials, including ceramics, metals, such as steel and aluminum, as well as composites of metals. Recently, improvements in armor plate have provided have a lighter-weight armor plate having greater ballistic protection. In this regard, maraging steel has been shown to provide certain advantages over various traditional forms of armor plate material.
Various compositions and methods of forming maraging steel for use in armor plate have been employed that are said to provide improved properties and resistance to penetration by projectiles. The hard front side of the armor plate is designed to break up or flatten the projectile, while the softer back side captures the projectile. For example, a dual hardness armor plate, commercially available as K12® from Allegheny Ludlum, Washington, Pa. is based on a Ni—Mo—Cr alloy steel that consists of a relatively high hardness front side and a softer back side. The assembly is roll bonded by a multi-step process that involves heating to a specific temperature and hot rolling until the two sides form a strong, metallurgical bond. The roll-bonded plates are then annealed, sheared, and flattened.
In another armor plate assembly and formation process, a composite plate of maraging steel includes a hard outer layer and a tenacious inner layer. The steel of the inner layer is produced in a process employing a chemical composition containing in percent by weight: C≦0.01, Si≦0.1, Mn≦0.1, P≦0.005, S≦0.005, Cu≦0.1, Mo is 4.80 to 5.20, Ni is 17.5 to 18.5, Cr≦0.1, Ti is 0.55 to 0.70, Co is 8.0 to 9.0. The steel of the outer layer is produced with purification in a process with a chemical composition containing in percent by weight: C≦0.01, Si<0.1, Mn is 0.02 to 0.20, P≦0.005, S≦0.005, Cu is 0.01 to 0.20, Mo is 4.80 to 5.20, Ni is 17.5 to 18.5, Cr is 0.01 to 0.20, Ti is 1.80 to 1.95, Co is 14.0 to 15.5, Al is 0.05 to 0.15, with the remainder being Fe and contaminants resulting from the manufacturing process.
Obtaining the alloy properties required under stringent consumer specifications requires careful and strict control of the alloying composition and forming processes. Even slight adjustments in processing of alloy compositions can significantly affect properties and performance of maraging steels. Due in part to these requirements, extensive efforts have been made to improve the mechanical properties of maraging alloys for various applications. For example, attempts to increase the yield strength and resistance to projectile penetration in maraging alloys have focused on alloy forming methods over a wide range of process parameters, including furnace time and temperature, annealing and age hardening temperatures, rolling operations, and the like. Reported methods use various processing conditions in an attempt to provide improved alloy properties to the maraging material.
Further improvements would be a welcome addition to the prior art processes in which control and monitoring of alloying elements and processing conditions are intended to improve the characteristics of maraging steels. More particularly, there is a continued need for approaches improving the yield strength in maraging steels and providing greater integrity, physical properties, and/or resistance to projectile penetration in the articles formed therefrom.